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Comet Burman Gearbox failure

Vic Youel

Well Known and Active Website User
Non-VOC Member
If you have followed the previous thread then you will appreciate the problems I have had with the Burman clutch.

Now my gearbox has expired. I had the engine rebuilt by Maughan's in 2008....no problem hear but the bike terminated in June in the Dolomites with a clutch problem .....awful design IMHO. With a new Conway clutch I thought all would be fine last Sat. With Bob Anderson and Paul Packman in pursuit I was "making good progress" however after 20 or so miles an awful screech at 60mph in top.......clutch in ....drifted to a halt. When I looked down I fully expected to see oil and bits of broken chaincase.

It worked in 1, 2 and 3rd gear but just a jangle of noise and neutrals in top gear... so got home at 50mph or so in 1,2 and 3 gears and the gearbox accumulated 4000 miles since I last disassembled it in 2008 to add BSF threads to the studs per Richardson.

It took me three hours minimum to remove the gear box and primary side and I discovered that the top gear selector had effectively formed braze on the top gear dog on the main shaft..... see attached pictures. (Takes me a day or two to reassemble as I did last week)

Everyone except Pete Deadman declared "lack of lubrication" as the cause at the Aldershot Section on Monday night and I was condemned for draining the oil and adding more grease (to cure the oil leaks). On closer examination it would seem the forked selector has been under pressure on one side trying to force the top gear into the final drive cog and accordingly welded itself and disintegrated.

On a visit (courtesy of Trevor Girton) to Maughan's yesterday, Steve had never seen anything like it. Later on arrival at the new VOCS shop in Kettering Ian Savage, Jenny Bloor and new storeman John Gunn seemed prepared to drop everything to help us.....sorry you never got to drink your cuppa Ian. They were extremely busy trying to get organised and shipping orders etc. Great team and excellent long-term facility.

Nothing was too much trouble for Ian and he empathised that he had experienced exactly the same problem with his Comet ...in his case it turned out to be main shaft bushes creeping in the main output gear. This is the normal first failure point on a Burman. In my case this turns out to be not the cause. So I immediately thought I had dislodged the circlip retaining the ball bearing at the selector end of the main shaft (Conways warn you about this potential problem when fitting their Honda clutch).....This was ok too.

Undoubtedly the selector fork is trying to press the top gear into mesh with the output sprocket main shaft and something is working against it. I cannot figure what is causing the interference. Could it be that the steel bush for the cam assembly is too deep into the casting?

The pin on the selector fork is rather worn as a result of the work it has tried to perform. This fork, incidentally, was replaced by the previous owner so there seems to be an endemic fault in the box. Possibly the the brazing/weld build up has occurred over my last 400o miles on the bike.....and Sat. was the final straw. Still I was lucky the whole machine did not just lock up!

Just a word of warning and sympathy for those who have had no practice in the art of assembling the kickstart and gear selector mechanism......if you think that is tricky (doing all this whilst the gasket cement is setting and the grease is dropping out) then don't get involved in sorting out a gearbox that may be an amalgam of differing parts.

All suggestions welcome

Vic

PS does anyone have an oil tight Burman box?

PPS VOCS do not stock the main shaft gear selector but Ian is on the job.

PPS Aren't Burman drawings and parts lists awful!
 

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rapcom

Well Known and Active Website User
VOC Member
A possible cause?

I don't suppose you inadvertently rode along with your right boot resting on the gear pedal ?
I know it sounds silly, but I have heard of it happening...
 

Robert Watson

Well Known and Active Website User
VOC Member
I had three catastrophic blow ups with a post war twin box. In the end it turned out that the engaging dogs were just plain worn out and trying to jump out of gear, and the only thing stopping it was the fork selector which had serious evidence or wear - as in 1/2 way through.
Perhaps this is your problem. Mine was when holding for a long stretch in second gear, perhaps yours is whilst in top. Look for evidence of worn dogs - if I recall without looking, on the end of the double gear that dogs into the output gear locking the input at the same speed as the output??

Or not!!

Robert
 

Vic Youel

Well Known and Active Website User
Non-VOC Member
Comet Burman Gearbox problems

Good morning all,

Robert, it could well be the cause as those internal teeth are pretty worn. Dick, I also think you have a point and I cannot be certain; however the gear lever has quite a long travel when selecting and I'm pretty certain that my foot relaxes back from the lever. Also something similar went wrong in the previous owner's hands.

An offer of spares made by a PM and a visit to another person offering parts and an examination (hopefully today) offer further encouragement.

Thanks everyone.

Vic
 

BlackLightning998

Well Known and Active Website User
VOC Member
Remember me Vic.....

Good morning all,

Robert, it could well be the cause as those internal teeth are pretty worn. Dick, I also think you have a point and I cannot be certain; however the gear lever has quite a long travel when selecting and I'm pretty certain that my foot relaxes back from the lever. Also something similar went wrong in the previous owner's hands.

An offer of spares made by a PM and a visit to another person offering parts and an examination (hopefully today) offer further encouragement.

Thanks everyone.

Vic

Hi Vic, best of luck.

If the spares you are offered on the PM just so happen to be atatched to a UFM, RFM and an engine plus some other bits, remember me who's looking for a ratty Comet to enjoy restoring myself (with the help of others!).

Stuart
 

Howard

Well Known and Active Website User
VOC Member
Good morning all,

Robert, it could well be the cause as those internal teeth are pretty worn. Dick, I also think you have a point and I cannot be certain; however the gear lever has quite a long travel when selecting and I'm pretty certain that my foot relaxes back from the lever. Also something similar went wrong in the previous owner's hands.

An offer of spares made by a PM and a visit to another person offering parts and an examination (hopefully today) offer further encouragement.

Thanks everyone.

Vic

Hi Vic

Hope I'm not teaching my Granny to suck eggs, but it's not the gear teeth that are the problem. They are straight cut, and won't cause the axial loading that has caused the damage, even when worn.

The axial (side) load as I see it can only come from "foot on the gear pedal" or "worn dogs" as mentioned by others. Your photos don't show the dogs, but you'll probably find that the original sharp corners are rounded off with wear and tear and the mating faces of the dogs have worn so that they are no longer parallel to the shaft. This causes a wedge action pushing the dogs out of mesh, if not restrained by the selector fork, which isn't designed for this type of loading.

I'd be surprised if it's only the top gear dogs that are worn, unless the gearbox has been partially rebuilt, so check all the dogs before rebuilding.

I've never had a grease filled Burman gearbox, but while it's apart I'd try to get the seals right, and fill it with a good gear oil instead. I'm sure someone will tell me why grease was used, and oil can't be substituted, but I've run a lot of oil filled Burmans with no problems that I remember.

H
 

Howard

Well Known and Active Website User
VOC Member
Hi again.

Just something that's been niggling me, and I've just looked at your first photo again - Why isn't the wear on the selector concentric with the bearing surface? It looks to me as though the selector was in the wrong place, for at least some of its life.

Perhaps someone can comment on that, I've never had to take a Burman apart.

H
 

Robert Watson

Well Known and Active Website User
VOC Member
Hi Vic

Hope I'm not teaching my Granny to suck eggs, but it's not the gear teeth that are the problem. They are straight cut, and won't cause the axial loading that has caused the damage, even when worn.
.

H

Actually in a standard Burman box, the ends of the gear teeth are the external dogs that fit into the internal dogs of the engaging gear. It was this part of the teeth to which I was refering.

Unless of course you have a Burman box wih dog type engagement which is a whole other story!
 

Vic Youel

Well Known and Active Website User
Non-VOC Member
Burman gearbox failure

Thanks for the comments everyone.... it is still puzzling me as to why the bronze deposit of the welded selector is on the side of the ring face of the sliding mainshaft gear.

Many other parts in the gearbox are very worn as well.....actually pretty knackered! A friend has donated loads of spare bits that are in better condition and I have decided to entrust my gearbox to an acknowledged expert engineer (who has further spares) and commissioned a total rebuild with oil seals on gear selector and kick start shaft. Combined with Peter Barker's sealed main bearing this will hopefully be very satisfactory and make an investment in a new shiny exhaust pipe worthwhile. An initial inspection by the expert did not produce a logical single reason for the failure.

I will report back in due course with any findings.

Vic
 

Howard

Well Known and Active Website User
VOC Member
Actually in a standard Burman box, the ends of the gear teeth are the external dogs that fit into the internal dogs of the engaging gear. It was this part of the teeth to which I was refering.

Unless of course you have a Burman box wih dog type engagement which is a whole other story!

You puzzled me with this one, until I realised my Comet didn't have a Burman gearbox (or an Albion) it had a much later Norton/AMC box with Manx gears (or so I was told). :confused:
Sorry, my mistake, problems of an ageing memory, but it does bear out what I said about oil filled Burmans (on several non Vincents) not giving me problems - the only Burman I had apart was an AMC..... please don't tell me AMC don't have dogs either, or I'll really have problems.

H
 

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